PROJECT SUMMARY Approximately 20% of breast cancer (BC) is caused by overexpression of the human epidermal growth factor receptor 2 (HER2), on the basis of which anti-HER2 therapies have been developed. Although these drugs have benefited BC patients, development of resistance and disease recurrence continue to be the major clinical challenges. These clinical problems warrant studies on alternative therapeutic strategies to treat HER2-positive BC. In this application, we propose to develop a new targeted therapy through pharmacological inhibition of the Src homology phosphotyrosyl phosphatase 2 (SHP2), which acts by blocking HER2 and switch oncogene overexpression. As opposed to the existing anti-HER2 drugs that act by inactivating the already expressed protein, targeting SHP2 has the potential to block the process of overexpression. We predict that targeting SHP2 that controls multiple signaling pathways is more effective and at the same time eliminates the need for developing drugs against multiple targets. In addition, targeting SHP2 can reduce toxicities associated with the use of drug cocktails and enormously cuts the cost of treatment. In line with this concept, we have invented a specific small molecule SHP2 inhibitor (WGMDY) that shows promising anti-cancer effects in nave and anti-HER2 drug resistant cells in culture and in vivo. In this study, we will investigate the potential of SHP2 as a drug target and WGMDY as an anti-cancer agent. The overall hypothesis is that pharmacological targeting of SHP2 induces remission of both treatment-nave and anti-HER2 drug resistant cells and tumors. To attain these goals, three specific aims have been proposed. First (Specific aim #1), we will determine the effect of pharmacological targeting of SHP2 on oncogene expression, cell proliferation, transformation, and cancer stem cell properties. Second (Specific aim #2), we will determine maximum tolerable dose, toxicity, bioavailability, and stability of WGMDY in vivo. And third (Specific aim #3), we will determine the anti-cancer activity of SHP2 targeting with WGMDY in primary spontaneous and xenograft models and in brain metastatic models. The outcome of the proposed study can show the potential of SHP2 as a drug target and the viability of the anti-SHP2 compound as a lead molecule for future development of anti-SHP2 drugs.